That always depends on the individual fic. I always have a single REASON for writing a fic, and it's usually some idea that the canon has sparked off in me. Meteorology was my need to tease out Arthur's motivations for behaving like a prat (because I love that he does, even though he's a very noble and good-hearted character, and I wanted to sort out in my own head how the inconsistencies in his behaviour made sense). Other stories arise from my desire to comment on a theme or a dropped plot point or the treatment of a character, or a what-if that fell into my head. There's always one thing.

What happens then is that a lot of my opinions about the canon find their way into the fic; these can be extra themes, extra views on relationships, anything. This happens especially in one-off fics for any fandom (like when I wrote for Pirates of the Caribbean and Prince Caspian) because I know that I don't really intend to play in that universe again, and so I use the one fic as a way to express everything I want to.